Chapter 7
Beyond alleles: quantitative genetics and the evolution of phenotypes
Continuous traits have a complex genetic basis
• Polygenic trait: influenced by many genetic loci
– Interaction between alleles (epistasis)
– Interaction with environment (phenotypic plasticity)
• Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits
Epistasis
• Epistasis is a phenomenon in which the expression of one gene depends on the presence of one
or more "'modifier genes'." A gene whose phenotype is expressed is called epistatic, while one
whose phenotype is altered or suppressed is called hypostatic.
•
Ex: Labrador retrievers
Phenotypic plasticity
• Phenotypic plasticity is the ability of an organism to change its phenotype in response to
changes in the environment.
• Phenotypic plasticity encompasses all types of environmentally induced changes (e.g.
morphological, physiological, behavioral) that may or may not be permanent throughout an
individual’s lifespan.
• It is NOT evolution.
• Ex: Daphnia
When components of variation are additive, genetic and environmental variance sum to total
phenotypic variance
• Heritability is the proportion of phenotypic variance due to genetic differences
• Broad sense heritability includes:
– Additive effects
– Dominance effects
– Epistatic effects
– Maternal/Paternal environmental effects
Natural Selection
The process in nature by which, according to Darwin's theory of evolution, only the organisms best
adapted to their environment tend to survive and transmit their genetic characteristics in increasing
numbers to succeeding generations while those less adapted tend to be eliminated.
• Natural selection is a mechanism that causes evolution. It occurs when some kinds of organisms
in a population leave more offspring than other kinds of organisms; the result is that the kinds of
organisms who leave more offspring will increase in frequency over time.
•
Natural selection can be understood as a logical argument in which a conclusion follows from a
set of premises (or a result follows from a set of initial conditions). Natural selection will operate
on anything that has the following properties:
• Reproduction
• New generation must exist
• inheritance
• “like must reproduce like”
•
•
variation in fitness
• Some individuals are more likely to reproduce
• The fitness of an organism, in its simplest evolutionary sense, is how many
successful offspring an individual has
variation in individual characteristic
• There must be variation in phenotype (ergo genotype)
Modes of selection
• Directional Selection
• Directional selection favors those individuals who have extreme variations in traits
within a population.
• A useful example can be found in the breeding of the greyhound dog. Early breeders
were interested in dog with the greatest speed. They carefully selected from a group of
hounds those who ran the fastest.
• From their offspring, the greyhound breeders again selected those dogs who ran the
fastest. By continuing this selection for those dogs who ran faster than most of the
hound dog population, they gradually produced a dog who could run up to 64km/h
(40mph).
• The greyhound was originally used to hunt the fastest of game, fox and deer. Their
breed dates to Egypt in 3BCE.
• Stabilizing selection
• Stabilizing selection favors the norm, the common, average traits in a population.
• Look at the Siberian Husky, a dog bred for working in the snow.
• The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs
have strong pectoral and leg muscles, allowing it to move through dense snow.
• If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they
would move slower or would sink and get stuck in the snow.
• Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds
and equipment, so the dog would have little value as a working dog.
• So stabilizing selection has chosen a norm for the size of the Siberian Husky.
• Disruptive selection
• Disruptive (diversifying) selection, like directional selection, favors the extremes traits in
a population. Disruptive selection differs in that sudden changes in the environment
creates a sudden force favoring that extreme.
• Think about the changes in the environment when that meteor crashed into Earth
65mya. A sudden decrease in light levels as the dust rose over large portions of the
Earth. Extremely large tidal waves washing miles over the land. Increased seismic
activity.
• The sudden lost of food along the coast, possible plague due to the high initial death
rate, dust filling the lungs of animals would have been the most stressful on larger
animals.
• Large animals need a large oxygen supply to supply energy to their muscles. They also
need a large, constant supply of food.
•
•
The sudden drop of oxygen due to the dust, and the drop in fresh food, large animals
would be stressed. If a plague started by the high death rate also hit these stressed
animals, they would have been sorely pushed to survive. Evidence shows that they did
not. So disruptive selection occurs quickly, selecting for those extreme traits that help
organisms survive in the new environmental conditions.
So disruptive selection occurs quickly, selecting for those extreme traits that help
organisms survive in the new environmental conditions.
Evolutionary response to selection
• How much the population changes depends on:
– Selection differential (S)
– Heritability
Selection coefficient
• The selection coefficient is a measure of the relative fitness of a phenotype. Usually denoted by
the letter s, it compares the fitness of a phenotype to another favored phenotype, and is the
proportional amount that the considered phenotype is less fit as measured by fertile progeny.
• s = 0 then is selectively neutral compared to the favored phenotype, while s = 1 indicates
complete lethality.
SO…
• Evolution and selection are not the same
– Selection can occur without evolution
• The magnitude of change depends on:
– Strength of selection (selection differential)
– Heritability
Quantitative trait loci (QTLs)
• Quantitative trait loci (QTLs) are stretches of DNA containing or linked to the genes that
underlie a quantitative trait. Mapping regions of the genome that contain genes involved in
specifying a quantitative trait is done using molecular tags, commonly SNPs.
• Quantitative traits refer to phenotypes (characteristics) that vary in degree and can be
attributed to polygenic effects, i.e., product of two or more genes, and their environment.
• E.g. Agouti